Finished AVI file wont open!
If an error happened before blender finished all the frames that were supposed to be rendered, I am afraid the .avi file may be corrupt. It cannot be opened or fixed. The way to render an animation the safe way is to always render as an image sequence. Meaning render each frame as a .jpg or .png file format. You would then use the VSE (Video Sequence Editor) in blender to combine all the images and save it as a .mov or .avi movie file format.

This way if some error occurs you can still restart the rendered at the point where the error occurred, instead of starting all over again because your movie file was corrupted by the incomplete rendering process.

That is how the animation industry does rendering work at a professional level too. This workflow allow for post production work, like adding visual effects and color correction after the all the frames are rendered.